Liquid developing method for electrophotography

ABSTRACT

Electrostatic latent images are developed in a liquid electrophoretic developer, and during the development, developer immediately adjacent the image whose toner concentration has become depleted is removed from the image area and replaced by fresh developer. Images are thereby more fully developed to the extent of the electrostatic charge, eliminating edge and halo effects that result from incomplete development and the preferential attraction of toner to areas of higher electrostatic charge over adjacent areas of lower charge.

United States Patent 11 1 Fukushima et al.

[ LIQUID DEVELOPING METHOD FOR ELECTROPHOTOGRAPHY [75] Inventors: Osamu Fukushima; Masamichi Sato; Seiii Matsumoto, all of Asaka, Japan [73] Assignee: Xerox Corporation, Stamford,

Conn.

221 Filed: Mar. 13, 1972.

211 Appl. No.: 234,154

[52] US. (:1 117/37 LE, 96/114, 118/637, 355/10 [51] Int. Cl C03g 13/10, C03g 15/10 [58] Field of Search 117/37 LE; 96/1 LY; 1l8/DIG. 23, 637; 355/10 [5 6] References Cited UNIT-ED STATES PATENTS 3,249,088 5/1966 Ostensen 117/37 LE 3,415,223 12/1968 Zweig 1 17 37 LE 3,642,5l5 2/1972 Sato et al. ll7/37 LE I I I I [451 Apr. 30, 1974 3,667,987 6/1972 Miller 117/37 LE 3,669,073 6/1972 Savit et al. 1 17/37 LE 3,627,557 12/1971 Sato et al ll7/37 LE Primary Examiner-William D. Martin I Assistant Examiner-M. Sofocleous Attorney, Agent, or Firm-James .l. Ralabate ABSTRACT Electrostatic latent images are developed in a liquid electrophoretic developer, and during the 'development, developer immediately adjacent the image whose toner concentration has become depleted is removed from the image area and replaced by fresh developer. Images are thereby more fully developed to the extent of the electrostatic charge, eliminating edge and halo effects that result from incomplete development and the preferential attraction of toner to areas of higher electrostatic charge over adjacent areas of lower charge.

3 Claims, 7 Drawing Figures LIQUID DEVELOPING METHOD FOR ELECTROIHOTOGRAPIIY INTRODUCTION AND SUMMARY INVENTION The present Invention relates to electrophoretic liquid development of electrostatic latent images such as are obtained in the field of electrophotography, and

more particularly the invention is directed to maintaining an adequate supply of developer toner at'the image surface, to facilitate complete development of various image charge levels, and to minimize or substantially eliminate edge effects normally experienced in the development of these images.

In the field of electrophotography the phenomenon of edge effect, or the preferential development of the peripheral portion of an' image charge area, is well known. To eliminate or correct this problem, it has been proposed to provide a development electrode in closely spaced facing relationship with the electrostatic image surface to be developed, and to provide liquid electrophoretic developer in the space between said electrode and surface. However, this method often results in insufficient developed image density, because it is difficult to maintain an adequate supply of fresh liquid developer in the very narrow space between the development electrode and the image surface to be developed. The liquid developer consists of colored toner particles suspended in an electrically insulating liquid vehicle, so that once the toner is exhausted from the minute supply of developer in said space, continued advancement of development is prevented. During the development that takes place under these circumstances, it will be appreciated that the central portion of a high charge area will receive toner until exhaustion of the toner supply in that region. At the edge of this high charge area however, toner will be received not only from developer in the immediate area, but toner will also be drawn from the developer in the region of an adjacent lesser charge image area. This effect causes the border of the high charge image area to be developed to a greater density than the equally highly charged central portion of this image area; and further, this action contributes to a halo effect around the high charge image area, in that the adjacent lower charge portion of the image is grossly underdeveloped because it is deprived of toner by the preferential attraction of toner from that area to the edge of the high charge image area.

It might be assumed that continued or prolonged development would rectify this situation, in that eventually sufficient toner would be deposited over the entire highly charged image area to completely satisfy the charge and provide a uniform density, and thereafter, toner would deposit in the halo area to a density appropriate to satisfy the lesser image charge in that area. However, prolonged development will not normally rectify the problem, because the developer immediately adjacent the image surface becomes essentially exhausted of toner, thus effectively terminating the development action. The interfacial adhesion between the image surface and the exhausted developer solution is such that replacement with fresh developer from the remaining developer solution is not accomplished even with stirring or agitation of the developer-to the extent that such agitation can be had without disturbing the toner already deposited in image configuration.

The purpose of the present invention is to overcome the foregoing disadvantages and substantially to eliminate or minimize the described edge and halo effects. Basically, the improved results of the present invention are accomplished by squeezing the exhausted developer layer adjacent the latent image carrier surface during the development process, whereby the exhausted developer solution is expressed away from the latent image surface, enabling fresh developer solution to replace the exhausted developer, and thus'pennitting continued development action. When development is caused to continue in this way, the highly charged latent image areas fully develop to a uniform image density until the charge is completely satisfied with toner, and once this occurs, the adjacent areas of lesser image charge develop to an image density appropriate to their respective levels of charge. Thus, the above-described edge and halo effects are eliminated, or significantly reduced. The expressing operation can be repeated an appropriate number of times during the development process, as may be required to effect full development of the electrostatic latent image.

It is therefore one object of the present invention to provide for the replacement of exhausted developer solution with fresh developer solution at the interface between an electrostatic latent image carrying surface and a liquid electrophoretic developer solution.

Another object of the present invention is to provide for said replacement by expressing the exhausted developer solution from adjacent said image carrying surface.

Still another object of the present invention is to provide for successive expressings of exhausted developer solution from adjacent said image carrying surface; to effect a substantially complete development of the electrostatic latent image.

Other objects and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description of exemplary specific embodiments of the invention, had in conjunction with the accompanying drawings, in which like'numerals refer to like or corresponding parts, and wherein:

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic vertical sectional view of a development apparatus embodying the principles of the present invention;

FIG. 2 is a perspective view of an expressing roll used in the apparatus of FIG. 1;

FIGS. 3AC are fragmentary views of the apparatus of FIG. 1 illustrating several modifications thereof;

FIG. 4 is a schematic vertical sectional view of another development apparatus embodying the principles of the present invention; and

FIG. 5 is perspective view of a guiding roll used in the apparatus of FIG. 4.

DETAILED DESCRIPTION Referring to FIG. 1, tank 13 contains a-conventional liquid electrophoretic developer 21 comprising toner particles suspended in an electrically non-conductive liquid. Paired squeeze rolls 14-15, l4'l5'; and 14"-l5" are mounted in tank 13 for rotation in the directions indicated by the arrows, so that an electrophotographic sheet 1 is fed between the paired rolls in the direction of arrow 2. Guides 17 and 17' are provided at the two ends of the tank 13 to direct sheet 1 into and out of the tank. The electrophotographic sheet I having an electrostatic latent image on one surface, is directed by guide 17 to pass through the nips of said squeeze rolls with the latent image facing upwardly, and is then directed by guide 17' upwardly out of the container 13. The electrostatic latent image on the surface of sheet 1 may be formed in a conventional manner, as by uniformly charging a photoconductive surface of sheet 1, and then exposing said surface to an optical image to discharge the light struck portion of the photoconductive surface.

A nozzles 16 is provided in advance of each pair of squeeze rolls14-15, 1415', and 14"-15" to direct liquid developer toward and over the latent image surface of sheet 1. By means of a pump, not shown, liquid developer 21 is continuously circulated from the tank 13 to the nozzles, thereby maintaining a constant supply of fresh developer flowing over the latent image area. The flow of liquid developer from the nozzles 16 is at a slow rate, since rapid flow would tend to deteriorate the existing development toner deposit. Said nozzles 16 can also be utilized as development electrodes by forming them of electroconductive material, and for that purpose it is preferable that the nozzles be flanged, as at 16a. The rolls 15, 15' and 15" are preferably made of electroconductive material in order to ground the rear or non-image side of photosensitive sheet 1 as the sheet advances through the developer. The rollers 14, 14' and 14" which are juxtaposed to the image surface of sheet 1 can be made of electroconductive or insulating material, or can have an electroconductive core covered with an insulation layer. These rolls can desirably be used as development electrodes to enhance the development results. The number of pairs of squeeze rolls may vary, as desired. Three pairs are shown, but it is suggested that the number may advantageously vary from two to five. A

The preferred shape for rolls 14, 14' and 14 is illustrated in detail in the perspective view of one roll 14in FIG. 2. The major portion 14b of the roll has a'uniform diameter, but the ends of the roll are formed to have a slightly larger diameter, providing a small annular flange structure 14a at each end of the roll. The enlarged diameter at 14a is grossly exaggerated in the drawings for illustration purposes, and in practice it is found that a radius enlargement between portions 141; and 14b is preferably only of the order of about 0.1 mm. When a sheet. 1 passes through the nip of rolls 14 and 15, the side edges of the sheet 1 are engaged between end flanges 14a of roll 14 and the surface of roll 15. Thus, in the nip of rolls 14-15 the lesser diameter portion 14b of the roll 14 is spaced a very small distance from the image surface of sheet 1. Rolls 14" and 14" are similar in construction and operation. Rolls 15, 15' and 15" function as backing rolls for rolls 14, 14 and 14". Obviously, an endless belt arrangement could be substituted for the rolls 15, 15 and 15", if desired.

Thus, in operation, as sheet 1 with'an' electrostatic latent image thereon enters the developer bath '21, it is directed by guide 17 to pass through the nips of the squeeze roll pairs 14-15, 14-15', and l4"-15", and

passes under the nozzles, 16 and is exposed to the developer flow issuing from these nozzles. The electrostatic latent image is partially developed as it passes under the first nozzle 16, and the developer solution immediately adjacent the image surface becomes largely depleted of toner particles. As the sheet proceeds into the nip of roll pair 1415, the close spacing of roll surface 14b to the image surface squeezes the depleted developer solution away from the surface of the' sheet, so that when thatportion of the sheet emerges from the nip of these rolls, fresh developer solution can again contact the image surface of the sheet and the development process can continue as the sheet passes under the second nozzle 16 and is exposed to the flow of developer solution emerging therefrom. Similar action occurs at the nip of roll pair l4'-15 and the third nozzle 16, and again at the nip of roll pair 14"15".

Because ofthe action of the squeeze rolls 14-15, l4'-15', and l415" in removing exhausted or toner depleted developer solution from immdiately adjacent the image surface of the sheet 1 and permitting further effective development action to continue, when the sheet. 1 energes from the developer bath 21 the image thereon is fully developed to maximum density, or nearly so, for the charge densities thereon. Consequently, for the reasons expressed previously, there are no, or very little edge and halo effects in the developed image. After emerging from the developer bath 21, the sheet 1 is dried of adhering developer liquid, and the toner image is fixed to the sheet or transferred to a receiving sheet in accordance with conventional practices in the art.

In place of the nozzles 16 in FIG. 1, alternative arrangements are shown in FIGS. 3A-C. In the latter Figs. nozzles 16 are replaced by nozzles 19, but the squeeze roll arrangement remains the same. FIG. 3A shows a modification in which the roll 14 is constantly cleaned of deposited toner by spraying the liquid developer from the nozzle 19 directly onto roll 14. In this case, the liquid developer sprayed onto the roll 14 flows over the roll into the developer bath to agitate the developer solution adjacent the surface of sheet 1. Obviously, the nozzle 19 need not be centered over the roll 14, but may be shifted toward one side or the other thereof. In FIG. 38 a separate development electrode 20 is provided between each set of squeeze rolls, and the flow of liquid developer from the nozzles 19 is directed against these electrodes and deflected toward the nips of the squeeze roll pairs. A further modification of this arrangement is illustrated in FIG. 3C, in which the electrode 20 is provided with sloping surfaces to deflect the developer flow from nozzles 19 more effectively toward the nips of the squeeze roll pairs.

A further embodiment of the invention is shown in FIG. 4 which is particularly suited for developing a long or indefinite length web 1a of photosensitive material, in lieu of the short sheet 1 in the FIG. lembodiment. This embodiment comprises a tank 30 containing conventional electrophoretic liquid developer 31. Rolls 37 and 38 are mounted adjacent one end of the tank for feeding the electrophotographic web 1a into the tank and are preferably provided with electrically insulating surfaces. These feed rolls can be wetted with the developer vehicle free of toner, to effect a prewetting of the web la prior to development to prevent deposition of toner from simple mechanical absorption of developer.

Within tank 30 are positioned three squeeze roll pairs 42-39, 43-40, and 44-41 which function in the same manner as, and correspond to the squeeze roll pairs 14-15, 14'l5, and 14"-l5" of FIG. 1. Nozzles 49, 50, 51 and 52 are also provided to generate a flow and agitation of developer solution, in the same manner as nozzles 16 in FIG. 1, or nozzles 19 in FIGS. 3A-C.

In addition, tank 30 includes a set of four roll type development electrodes 32, 34, 35 and 36. One roll 32 is shown in enlarged perspective in FIG. 5. It includes a primary cylindrical portion 32b, with enlarged diameter flanges 32a at each end. These rolls are formed of conductive material, to enable them to perform the development electrode function.

As web 1a passes through developer solution 31, its photoelectric latent image surface faces upwardly toward rolls 32, 34, 35 and 36, and toward rolls 42, 43 and 44. The longitudinal side edges of the web lie in contact with the flanges 32a of roll 32, and the corresponding end flanges of rolls 34, 35 and 36. Thus, developer solution enters in the space between the image surface of the web 1a and the surface of the reduced diameter portions of the development electrode rollsv 32, 34, 35 and 36. Development proceeds in stages in the manner described above with respectto FIG. 1, and exhausted developer solution is squeezed from the surface ofthe web la with each pass through the nip of a squeeze roll pair. As the web la emerges from the development tank 30, excess developer solution is removed by press rolls 45 and 46, and the web with its developed image is then driedand fixed in a conventional manner, with or without transfer of the image to a separate receiving sheet, as desired.

Thus, it will be appreciated that a primary feature of the present invention is the periodic removal of exhausted or depleted developer solution from adjacent the image surface of the image carrier, and replacement with fresh developer containing an adequate concentration of toner particles. In this manner, the electrostatic latent image is caused to develop to the full densities represented by the different levels of electro- 6 the edge and halo effects ordinarily encountered. The foregoing embodiments of the invention are only illustrative of the invention, and various modifications and alternatives will be apparent to those skilled in the art. Accordingly, the scope of the present invention is not to be construed as limited to these embodiments, but as including such modifications and alternatives as are embraced by the spirit and scope of the appended claims.

We claim:

1. A method of developing an electrostatic latent image on the surface of a carrier with liquid electrophoretic developer containing toner particles susprising immersing said surface in a quantity of said developer and causing development of said latent image to proceed whereby the concentration of toner in the volume of developer immediately adjacent said image surface is depleted, removing depleted developer from adjacent said image surface by feeding and squeezing the immersed carrier by means of squeeze rolls to express depleted developer from the latent image surface I while the image is undergoing development, exposing static charge thereon, thereby effectively eliminating surface while the image is undergoing development, and replaced with fresh developer by flowing fresh developer over the surface, and development of the latent image is continued.

3. A method as set forth in claim 1, wherein removal of depleted developer is effected by squeezing the depleted developer away from adjacent said image sur-' face by passing said carrier through the nip of squeeze rolls. 

2. A method as set forth in claim 1, wherein depleted developer is again removed from adjacent said image surface by additionally squeezing the carrier to express additional depleted developer from the latent image surface while the image is undergoing development, and replaced with fresh developer by flowing fresh developer over the surface, and development of the latent image is continued.
 3. A method as set forth in claim 1, wherein removal of depleted developer is effected by squeezing the depleted developer away from adjacent said image surface by passing said carrier through the nip of squeeze rolls. 